One example of a power plant employing an axial flow rotary machine is a turbofan gas turbine engine for powering an aircraft and the engine's associated nacelle. The nacelle shelters the engine and accessory equipment, and provides aerodynamic surfaces which cooperate with the engine for generating thrust.
A primary flow path for working medium gases extends through the central or core region of the turbofan engine. A secondary flow path for working medium gases is disposed outwardly of the primary flow path and is annular in shape. The engine has a plurality of fan blades which extend radially outwardly across the primary flow path and secondary flow path. These fan blades pressurize working medium gases entering both flow paths of the engine. The ratio of mass flow through the secondary flow path to the mass flow through the primary flow path is the by-pass ratio of the engine. By-pass ratios greater than 3.5 are referred to as high by-pass ratio turbofans.
The nacelle of high by-pass ratio turbofans includes a fan nacelle and a core nacelle which are of relatively large diameter. These nacelles are often referred to as the fan cowling and the core cowling. The core nacelle is spaced from the engine leaving a core compartment therebetween which extends about the gas turbine engine. As a result of heat transfer from the gas turbine engine and many accessories disposed in the compartment, the temperature of gases in the core compartment may exceed one-hundred and fifty degrees Fahrenheit.
The core nacelle is disposed radially inwardly of the fan nacelle leaving a region therebetween for the secondary flow path. An exterior wall of the core nacelle and an interior wall of the fan nacelle bound the secondary flow path. Thus, as these gases are flowed through the engine, the gases flow over the walls of the nacelles and the walls are contoured to minimize the drag effect that these walls have on the high velocity gases in the secondary flow path.
The turbofan engine includes a compression section, a combustion section and a turbine section. The primary flow path extends axially through these sections of the engine. The working medium gases are drawn into the compression section where they pass through several stages of compression, causing the temperature and pressure of the gases to rise. The gases are mixed with fuel in the combustion section and burned to form hot pressurized gases. These gases are a source of energy to the engine and are expanded through the turbine section to produce work.
The engine has a rotor assembly for transferring the work of compression from the turbine section to the compression section. A stator assembly extends circumferentially about the rotor assembly to circumscribe the rotor assembly and axially through the engine. The stator assembly includes an outer case or pressure vessel which confines the high pressure working medium gases to the primary flow path.
The rotor assembly includes rows of rotor blades which extend outwardly across the working medium flow path. The stator assembly includes a row of stator vanes which extends radially inwardly across the working medium flow path upstream of each row of rotor blades to direct the working medium gases at the proper angle into the rotor blades.
The stator assembly includes sealing elements for blocking the leakage of working medium gases from the working medium flow path. The sealing elements in modern engines include outer air seals formed of arcuate segments which extend circumferentially about the interior of the engine. The sealing elements are spaced radially from the rotor blades by a clearance gap G. It is important to avoid predictable local discontinuities in the circumferential shape of the outer air seal to insure the predetermined clearance gap is minimized. It is also important to minimize unpredictable discontinuities in the circumferential shape of the outer air seal to avoid destructive interference between the rotor blade tips and the outer air seal, which can degrade the sealing ability of the outer air seals.
Modern high bypass turbofan engines employ clearance control systems to change the diameter of the outer air seal to minimize the clearance gap at steady state operating conditions while accommodating transient differences in growth between the diameter of the outer air seal and the diameter of the row of rotor blades. U.S. Pat. No. 3,966,354 and U.K. Pat. No. GB2025536 are exemplary of structures using cooling air or heating air on the interior of the outer case (that is, inside the pressure vessel) to control the diameter of the outer air seal and, thus, the clearance gap between the outer air seal and the rotor blades of the rotor assembly.
More particularly, U.S. Pat. No. 3,966,354 issued to Patterson entitled "Thermal Actuated Valve for Clearance Control" shows an outer case which forms the pressure vessel for the engine and an inner case spaced radially inwardly from the outer case. The outer air seal is attached to the inner case. A chamber is disposed between the inner case and the outer case and receives cooling air from a compression section of the engine. The cooling air is flowed through the chamber and through flanges on the inner case to control the clearance between the rotor assembly and the outer air seal. As will be appreciated, the cooling air must be at a high pressure because the cooling air is exhausted inside the pressure vessel, such as into the working medium flow path. A significant amount of work must be done on the gases to compress the gases to a pressure which will permit the gases to flow into the working medium flow path. And, compressing the gases to the high pressure causes the temperature of the gases to rise decreasing the effectiveness of the cooling air as a cooling medium.
U.K. Pat. No. GB 2,025,536 issued to Davison entitled "Turbine Rotor-Shroud Clearance Control System" is another example of an internal clearance control system. Davison shows an outer case, a shroud support inwardly of the outer case and an outer air seal attached to the shroud support. An impingement ring extends circumferentially about the shroud support to impinge cooling air on the shroud support to adjust the clearance gap between the outer air seal and the rotor blade. Cooling air is flowed at high pressure from the chamber between the impingement ring and the inner case through a flange joint to a downstream location. As in the Patterson patent the cooling air is discharged inwardly of the pressure vessel of the gas turbine engine into the working medium flow path.
In other modern engines, the clearance gap between the rotor blades and the outer air seal is adjusted by attaching the outer air seal to the pressure vessel, such as a coolable outer case, and impinging cooling air on the exterior of the case. Examples of engines employing the coolable outer case to adjust the clearance between rotor blades and the outer air seal are shown in U.S. Pat. No. 4,019,320 issued to Redinger et al entitled "External Gas Turbine Engine Cooling for Clearance Control"; U.S. Pat. No. 4,247,248 issued to Chaplin et al entitled "Outer Air Seal Support Structure for a Gas Turbine Engine"; U.S. Pat. No. 4,485,620 issued to Koenig et al entitled "Stator Assembly for a Gas Turbine Engine"; and, U.S. Pat. No. 4,533,901, issued to Laurello entitled "Stator Structure for a Gas Turbine Engine". As shown in these patents, the outer case is attached to the outer air seals so that selective cooling of the outer case changes the diameter of the outer case and causes a similar change in the diameter of the seals. Thus, as the diameter of the outer case decreases, the outer air seal diameter decreases and the clearance gap becomes smaller.
In each of these patents, the outer air seal is attached to the outer case either at one location (Laurello) or at two locations (Redinger, Chaplin, and Koenig). The outer case includes a coolable rail which extends circumferentially about the exterior of the outer case and extends outwardly into a nacelle compartment. The rails stiffen the case and increase the force exerted by the outer case for a given change in temperature of the rails. The rails may be flanges which are bolted together or an integral rail which extends as one piece about the exterior of the outer case.
Arrays of cooling tubes extends circumferentially about the engine and are in flow communication with the compression section of the engine, such as the fan section of the engine, to provide cooling air at a pressure which is relatively low in comparison to the cooling air used in internal clearance control systems. The cooling air has the advantage of being cooler by reason of the amount of pressurization because the pressure in the nacelle compartment is much less than the pressure on the interior of the engine.
Because the cooling air that is impinged on the rails is removed from the working medium flow path after energy is expended by the engine to pressurize the gases, it is desirable to reduce the amount of cooling air needed for clearance control. In addition, it is desirable to operate the engine with smaller clearances between the rotor blades and the outer air seal to minimize the amount of cooling air needed to move the outer case from its maximum diameter to its minimum diameter.
Accordingly, scientist and engineers working under the direction of applicant's assignee have sought to develop constructions which would require less cooling air for a given amount of clearance control to increase the efficiency of the engine while maintaining an adequate fatigue life in components of the engine.